In the related art, with development of communication techniques, an optical network that realizes high-speed transmission using an optical fiber has become popular. Normally, the optical network is configured with an OLT (Optical Line Terminal) as an optical transmission apparatus set on the common carrier side, an ONU (Optical Network Unit) as an optical subscriber apparatus set on the user side and an optical fiber cable connecting these. Although a plurality of schemes are applicable to the optical network, in view of simplification and lower cost of the network configuration, a PON (Passive Optical Network) scheme is generally adopted. The PON scheme realizes single-fiber bidirectional optical transmission and reception to transmit light in two directions between the OLT and the ONU by dividing one optical fiber by an optical splitter and wiring the result for the user.
A single-fiber bidirectional optical transceiver has a bidirectional optical transmission/reception device. The bidirectional optical transmission/reception device is connected to a circuit board via an FPC (Flexible Printed Circuit). In the bidirectional optical transmission/reception device, an LD (Laser Diode) for optical transmission and an APD (Avalanche Photo Diode) or PD (Photo Diode) for optical reception are mounted in one chassis. Especially, since an optical transceiver used for optical transmission in the PON scheme corresponds to single-fiber bidirectional optical transmission/reception, an FPC for transmission and an FPC for reception are mounted in the identical chassis.
[Patent Literature 1]    Japanese Laid-open Patent Publication No. 2010-008673
However, recently, in ITU (International Telecommunications Union), an XGPON scheme supporting higher-speed transmission of 10 Gbps than a GPON (Gigabit Passive Optical Network) scheme in the related art was standardized. In the XGPON scheme, a crosstalk between an electric signal of about 2.5 V flown in the FPC for transmission and a weak electric signal of about 10 mV flown in the FPC for reception becomes significant especially, and therefore it is difficult to maintain a miniature chassis while performing stable optical transmission/reception in the identical chassis. Also, at the time of connection between a PD for optical reception and a circuit board, there is a case where a micro-strip line formed on the front surface of an FPC for reception touches a wall surface of a chassis. Accordingly, a characteristic impedance set to a predetermined value (e.g. 50Ω) changes. Such an impedance change is a cause of degradation of optical transmission quality.